1. Field of the Invention
The present invention relates generally to decoders for subsampled video signals, and more particularly to a decoder for a subsampled video signal which is band compressed by the interframe offset subsampling. This invention has particular applicability to a MUSE decoder for a video signal which is band compressed based on Multiple Sub-nyquist Sampling Encoding (hereinafter referred to as MUSE) More specifically, this invention relates to an improvement in an interframe interpolation circuit for decoding a still picture signal, provided in the MUSE decoder.
2. Description of the Background Art
Various television broadcastings for transmitting high-quality video have been proposed in recent years. Nippon Hoso Kyokai (NHK) in Japan has proposed a high-definition television system called high vision. According to the standard of this high vision, the number of scanning lines is 1125, a field frequency is 60 Hz, an interlace ratio is 2:1, and a length-to-breadth ratio of a picture is 9:16. A baseband signal of this high vision signal has a bandwidth of 22 MHz for a luminance signal and 7 MHz for each of two color difference signals (R-Y, B-Y).
Since the high vision signal includes signal components of a wide band, it cannot be transmitted as it is by using a bandwidth (27 MHz) on one channel of an usual satellite broadcasting. Therefore, NHK has proposed a band compressed transmission system for converting the high vision signal into an 8 MHz signal. This band compressed transmission system is called Multiple Sub-nyquist Sampling Encoding (MUSE system). An application of this MUSE system enables the bandwidth of the high vision signal to be compressed to 8 MHz and thus be transmitted on one channel band of the satellite broadcasting. A general description concerning the MUSE system is disclosed, for example, in U.S. Pat. No. 4,692,801 assigned to Ninomiya et al on Sept. 8, 1987. In addition, another description with the MUSE system is found in an article entitled "MUSE: Transmission System of High Vision Broadcast via Satellite" in Nikkei Electronics, Nov. 2, 1987, pp. 189-212. As described in those articles, it is noticed that the MUSE system is a band compression technology employing correlation properties of a video signal.
FIG. 1 shows sampling points of the high vision signal and those points in each field thereof. In this figure, marks of a hollow circle (.circle.), a hollow square (.quadrature.), a solid circle (.circle.) and a solid square (.quadrature.) represent sampling points in the 4n-th, the (4n+1) -th, the (4n+2) -th and the (4n+3) -th fields, respectively. T.sub.0 represents a sampling interval, which corresponds to the reciprocal of a transmission sampling rate (16.2 MHz). In sampling the high vision signal, a sampling phase is offset between any fields, between any frames and between any lines so that the sampling points do not overlap one another between fields, between frames and between lines. That is, the sampling phase is controlled so as to be circulated every four fields, so that a MUSE signal is generated by sampling of the high vision signal.
A MUSE decoder for decoding this MUSE signal to the original high vision signal carries out different processings for a signal in a still picture portion and a signal in a motion picture portion.
In the processing of the still picture portion, since a picture has a correlation between any fields and between any frames, a pixel which drops out between any pixels being transmitted at present is interpolated based on a pixel one field before, a pixel one frame before and a pixel three fields before. That is, a video in the still picture portion is reproduced based on the MUSE signal to be inputted during the four-field period.
Meanwhile, in processing of the signal in the motion picture portion, there exists no correlation with time, i.e., no correlation between any fields and between any frames. Thus, a reproduction is carried out only with pixel data at a sampling point in the field, which is being transmitted at present. In addition, the pixel dropping out between the pixels being transmitted at present is interpolated utilizing a correlation between lines, namely, pixels on at least the upper and lower lines.
The signal processing in the conventional MUSE decoder as described above will now be described with reference to FIGS. 2 and 3. FIG. 2 shows a MUSE signal transmission system, and FIG. 3 is a schematic diagram of the MUSE decoder, which is simplified to facilitate the description thereof.
Referring to FIG. 3, the MUSE decoder comprises a MUSE signal input terminal 10, an 8.15 MHz low-pass filter 11, an A/D converter 12 for sampling pixel data in response to a clock signal of 16.2 MHz, a still picture processing circuit 13, a motion picture processing circuit 17, a motion detecting circuit 20, a mixing circuit 21, a TCI decoder 22 and a synchronization/control signal detection circuit 23. The still picture processing circuit 13 comprises an interframe interpolation circuit 14, a sampling frequency converting circuit 15, and an interfield interpolation circuit 16. The motion picture processing circuit 17 comprises an intrafield interpolation circuit 18 and a sampling frequency converting circuit 19.
The mixing circuit 21 mixes a signal from the still picture processing circuit 13 and a signal from the motion picture processing circuit 17. A mixing ratio thereof varies depending on the amount of motion of a video detected by the motion detection circuit 20. A high vision signal is outputted through the TCI decoder 21. The synchronization/control signal detection circuit 23 carries out (a) detecting a horizontal/vertical synchronizing signal, (b) generating clock signals with various frequencies (16.2 MHz, 32.4 MHz, 48.6 MHz and the like), (c) detecting control signals having motion vector data or the like shown in Table 1 in the following, and (d) generating a control signal and a clock signal of each circuit based on the detection of those control signals.
TABLE 1 ______________________________________ BIT NO. CONTENT OF CONTROL ______________________________________ 1 Interfield subsampling (1: when sampling points phase (Y) are on the right) 2 Horizontal motion (Positive: when the picture vector (2') shifts to the right) 3 #2, LSB 4 ICK unit of 32 MHz 5 6 Vertical motion (Positive: when the picture vector (2') shifts downwards) 7 #6, LSB 8 Line unit 9 Y subsampling phase (1: when sampling points are on the right on odd number lines) 10 C sampling phase (1: when the value of line #2 (fraction is discarded) is an odd number and sampling points are on the left) 11 Noise reduction is carried out in response to the 12 value of noise reduction control 13 Interlace flag 14 Motion detection (1: when the lower sensitivity control (1) sensitivity is selected) 15 Motion detection sensitivity control (2) 16 0: normal 17 Motion information 1: completely still picture 2: not completely still picture 18 3.about.7: the degree of motion 19 None 20 AM/FM (1: AM, no emphasis) 21.about. Spare 32 ______________________________________
After the MUSE signal inputted is converted to digital data by the A/D converter 12, the still picture processing circuit 13 and the motion picture processing circuit 17 carry out decoding for a still picture and a motion picture, respectively. Output signals from these circuits 13 and 17 are mixed together at the mixing circuit 21 depending on the amount of motion detected by the motion detection circuit 20. An output signal of this mixing circuit 21 is applied to the TCI decoder 22, so that the TCI decoder 22 outputs a high vision signal.
FIG. 4 shows one example of the interframe interpolation circuit 14 and the intrafield interpolation circuit 18 in the conventional MUSE decoder. In this example, the processing at the intrafield interpolation circuit 18 is time-consuming. Therefore, in order to correct a large deviation between the timings of signals in the still picture processing circuit 13 and the motion picture processing circuit 17, an output signal of the interframe interpolation circuit 14 is delayed through one-line memories 24a and 24b in the intrafield interpolation circuit 18, so that a signal, the timing of which is controlled by this delay, is outputted to the sampling frequency converting circuit 15.
Referring to FIG. 4, a pixel signal Sa in the present field as shown in FIG. 5 is inputted into a terminal a of a switch S1 for interframe interpolation. (Each pixel signal in the present field is denoted with "P" in the following description and the following figures). Meanwhile, a signal Sb in which pixel signals 4 two frames before are interpolated between pixel signals 2 one frame before as shown in FIG. 5, is inputted into the other terminal b of the switch S1. The switch S1 operates responsive to an output signal (subsampling clock) Se from an EXOR circuit 39 to output a signal Sc in which the pixel signals P included in the signal Sa are interpolated between the pixel signals 2 one frame before in place of the pixel signals 4 two frames before. A frame memory 26 for delaying the input signal Sc by approximately one frame period is provided in the interframe interpolation circuit 14. The frame memory 26 comprises field memories 27 and 28 each constituting one field delay circuit. This one-field memory 28 has its delay time controlled responsive to a motion vector signal in order to correct a motion vector.
A motion detecting circuit 20' receives the respective signals in the present frame, one frame before and two frames before. As mentioned above, since the sampling points of the MUSE signal are circulated every two frames (four fields), the motion detecting circuit 20' detects a motion by comparing the pixel signals in the present frame and those two frames before (the detection of the difference in motion between every two frames). Since the motion detection is incomplete only by detecting the difference between every two frames, the detection circuit 20' also detects a motion by comparing the pixel signals in the present frame with those one frame before. This motion detection between any frame is carried out by comparing signal components equal to or less than 4.2 MHz, which have no folding distortion generated by subsampling in the still picture. The signals, which represent the amount of motion detected by these two motion detecting operations, are applied to the mixing circuit 21, and the mixing ratio is controlled as described above.
The clock signal of 16.2 MHz is applied to the EXOR circuit 39 through an input terminal 29. A phase control signal for interpolating the pixel signals in the present frame in place of the pixel signals two frames before by the switch S1 is applied to the EXOR circuit 39 through the other terminal 30. This phase control signal is generated in the synchronization/control signal detecting circuit 23 in response to 9th bit data in a control signal and a horizontal/vertical synchronizing signal.
As is known, the intrafield interpolation circuit 18' of the MUSE decoder produces pixels dropping out between the transmitted pixels by filtering, and also filters the transmitted pixels, resulting in an enhancement in the degree of freedom in a frequency characteristic of a video signal. This intrafield interpolation circuit 18' comprises line memories 24a, 24b, 24c and 24d for one horizontal scanning period (1H) delay, switches S2a, S2b, S2c, S2d and S2e for selecting the pixel signals in the present frame and inserting a ground signal (0 signal) in place of the pixel signals one frame before, and one-dimensional transversal filters 32a, 32b, 32c, 32d and 32e. These one-dimensional transversal filters 32a-32e are identical in their configurations, but different from one another only in their tap coefficients to be set. The transversal filter 32a comprises delay elements 34 for delaying by a time period corresponding to one pixel. Each of these unit delay elements 34 is constituted by a D type flip-flop (D-FF). Multipliers 33 are provided in the transversal filter 32a. The multipliers 33, each constituted by a ROM in general, multiply signals delayed by the delay elements 34 by the respective tap coefficients different from one another. An adder 36 is
to respective outputs of the transversal filters 3a-32e. A signal, the result of the addition by the adder 36, is outputted through a terminal 37 to the sampling frequency converting circuit 19 for motion picture signal processing.
A recursive noise reduction circuit has conventionally been well known as a noise reduction circuit. When this recursive noise reducer is employed in the MUSE decoder, sampling points of the MUSE signal coincide with one another every four fields (two frames), and hence a recursive noise reduction with interframe correlation is carried out. The noise reduction circuit provided in the interframe interpolation circuit 14' uses a frame memory 26 for the interframe interpolation circuit 14' as a 2-frame delay element, as shown in FIG. 6. Referring to FIG. 6, the noise reduction circuit provided in the interframe interpolation circuit 14' comprises a subtracter 44 for detecting a difference signal between two frames, a ROM (Read Only Memory) 40 for multiplying a coefficient, and an adder 42 for noise reduction. The ROM 40 outputs a signal for reducing noise.
The noise reduction can be carried out basically only when the correlation exists between the frames. Therefore, the recursive noise reduction circuit between two frames is employed only for the still picture signal processing, as is already known. In the conventional circuit shown in FIG. 4, however, an output signal subjected to the noise reduction is applied through a signal line L4 to the intrafield interpolation circuit 18' for motion picture processing. This causes an influence of the noise reduction to be exerted on a motion picture, which is not preferable. In order to reduce this influence, an effect due to the recursive noise reduction is determined smaller in the conventional circuit shown in FIG. 4. As another counter-measure substituting for the determination of this smaller effect, a dedicated delay circuit 24' is separately provided for delaying an output signal of the interframe interpolation circuit 14', as shown in FIG. 6. Consequently, delay circuits 24a and 24b in the intrafield interpolation circuit 18' need not be used, and hence the noise reduction does not affect the motion picture. However, an additional delay circuit 24' is required instead, leading to an increase in the occupied area on a semiconductor chip.
The related art of interest to the present invention is disclosed in Japanese Patent Laying-Open No. 62-53081, entitled "MUSE System Television Receiver". This related art is considerably different from the present invention in its circuit configuration, but discloses a noise reducer based on the MUSE system.